In chess, and presumably other games where two players alternate
turns, the word move has two meanings:
- One player moving one of his pieces
- White moving a piece, and Black's move in response
Presumably because the game can end with White making the final
move (I can't think of any other reason this term would
be necessary), the technical term for the first
of these meanings is ply. Thus, a chess game in which
White moves twenty one times and Black moves twenty times
is said to comprise forty one plies.